Gas turbine engines are typically started by the use of an externally applied cranking torque. In the usual case, an electric, hydraulic or pneumatic starter motor is coupled via the usual power take off and gear box to the shaft of the gas turbine. Starter motors of this sort are sized to accelerate the gas turbine engine to a so-called "light-off" speed whereat fuel is injected into the combustor of the turbine and ignited.
The resulting gases of combustion then initiate an acceleration process, accelerating the turbine wheel and compressor of the gas turbine engine, still with the assistance of the externally applied torque from the starter. This is continued until such time as a self-sustaining speed is reached, at which time, the externally applied torque from the starter may be removed.
There is, of course, a transition as the turbine progresses from an unfired to a fired state. During this transition, there is a corresponding change intorque within the turbine itself As may be ascertained by reference to FIG. 1, which illustrates a plot of torque against shaft speed as a percent of normal operating speed in the starting of a typical gas turbine, as the gas turbine is accelerated from standstill, an increasing resisting torque due to the resistance to cranking is present. The torque produced by a typical starter is, at the same time, decreasing At the time of ignition, after a short transition, there is a change in torque from a positive or resisting cranking torque to negative or accelerating torque as the gases of combustion resulting from ignition begin to drive the turbine.
A so-called "pinch point" represents the difference in torque between that provided by the starter and the resisting or positive cranking torque of the engine just prior to ignition. This "pinch point" effectively dictates the sizing of the starter motor and its energy requirements. That is to say, the starter must be sized and provided with enough energy to accelerate the turbine past the ignition point so that the engine operation will move to the negative torque position to achieve acceleration toward self-sustaining speed.
Frequently, sizing the starter to accomplish this job results in excessive torque capacity within the starter for all, but operating conditions right around the pinch point. This, of course, translates to additional bulk and weight in the starter system and neither is desirable in many applications, particularly in aircraft applications In such applications, additional bulk may increase the frontal area of an engine nacelle or the like thereby increasing drag while an increase in weight will reduce the useful load that may be carried by the aircraft.
Where the starter is sized somewhat smaller to avoid these difficulties, the torque output may be marginal at the pinch point and result in a so-called "hung start" where combustion is achieved within the gas turbine but the same will not accelerate.
The present invention is directed to overcoming one or more of the above problems.